1. Technical Field
The present invention relates generally to liquid crystalline light modulating devices, and more specifically to new phase-separated polymeric-liquid crystalline display cells and materials which exhibit different optical states under different electrical field or mechanical stress conditions and are characterized by a unique combination of properties, including optical multistability and haze-free light transmission at all viewing angles in either a field-ON or field-OFF mode.
2. Description of the Related Art
Electrically switchable liquid crystal-polymer films intended for use in various electro-optical devices have been prepared by mechanical entrapment procedures. One such technique involves imbibing liquid crystal into micropores of a plastic or glass sheet. Another technique involves evaporation of water from an aqueous emulsion of nematic liquid crystal in a solution of water-soluble polymer such as polyvinyl alcohol or in a latex emulsion.
A different procedure offering significant advantages over mechanical entrapment techniques and the emulsification procedure involves phase separation of nematic liquid crystal from a homogeneous solution with a suitable synthetic resin to form a liquid crystal phase dispersed with a polymer phase. The resulting materials are referred to as polymer dispersed liquid crystal (PDLC) films. Some of the advantages of PDLC films are discussed in U.S. Pat. Nos. 4,671,618; 4,673,255; 4,685,771; and 4,788,990; the disclosures of which are incorporated by reference. PDLC films have been shown to be useful in many applications ranging from large area displays and switchable coatings for windows to projection displays and high-definition television.
The methods of phase separation can be carried out by polymerization initiated by addition of a curing agent, by ultraviolet light or by cooling into the region of immiscibility. Another method is evaporating a solvent from a matrix-producing composition of a solution of polymer and liquid crystal in the solvent.
In windows or displays as described above in which the ordinary index of refraction of the liquid crystal is matched to the refractive index of the polymer, the device appears most transparent (field-ON-state) when viewed along the direction of the field which is usually normal to the viewing surface. Transparency decreases giving rise to increasing "haze" at increasing oblique viewing angles until an essentially opaque appearance is detected at an oblique enough angle. This condition of haze results from the fact that the farther the viewing angle is from the orthogonal, the greater is the perceived mismatch between the effective index of refraction of the liquid crystal and the refractive index of the matrix.
A further development of PDLC films disclosed in U.S. patent application Ser. No. 07/324,051, now U.S. Pat. No. 4,994,204, issued Feb. 19, 1991, involves the use of a birefringent polymer, e.g., a liquid crystal polymer. The PDLC film prepared with the birefringent polymer has the characteristic of displaying haze-free transparency for all directions of incident light. This is accomplished by matching the ordinary and extraordinary indices of refraction of the polymer to the ordinary and extraordinary indices of refraction of the liquid crystal.
PDLC films made with birefringent polymer can operate in the normal manner so that they are clear in a field-ON state and light scattering in a field-OFF state. Alternatively, the films can be made to operate in a reverse or "fail-safe" mode such that the material is clear in the absence of a field and is light scattering in the field-ON state.
It is also known to provide liquid crystal displays wherein a pen or stylus is used to change liquid crystal material from a planar state to a focal conic state or vice versa. These pen-based displays are employed as reusable, erasable writing tablets in such applications as children's toys and in portable electronic devices. As disclosed in U.S. Pat. No. 4,525,032, which is incorporated by reference, these devices employ cholesteric or smectic liquid crystal material that is changeable between states by the application of pressure, heat, light or an electric field. The writing on these tablets may then be erased by flexing the display, by applying an electric field or by exposing the material to an appropriate heating or cooling temperature.